Public health considerations and medical evidence concerning resuscitation practice in case of heart fibrillation motivated initiatives of installing Automatic External Defibrillators (AEDs) as public-access emergency medical devices in places with high public frequency such as train stations, airports, shop malls etc. Originally a high-tech medical device in the hands of experts the defibrillator had to be reshaped for layman use in the public sphere. The goal of the study is to reconstruct the history of the development of the public-access device in relation to the evaluation criteria. It can be shown that a great deal of the initial controversy was associated with the notorious neglecting of the distinction between efficacy assessment (with respect of the device performance under controlled conditions) and effectiveness assessment (with respect of the performance under every-day-life conditions). In order to overcome the laymen deficit in handling the medical instrument the technology design stressed a communication form based on evoking expert knowledge by oral recommendations to the user through the device itself. The unknown user was replaced by the dictated layman dummy constructed by the expert discourse. The further choices concerning the framing of the installation vary according to the negotiations among the interest groups involved in the decision procedure. In most cases the compromise consists in concepts that presuppose a limited group of potential users with a minimum of experience or special defibrillation training.